Polyethylene molding compound suitable as a pipe material with excellent processing properties

ABSTRACT

The invention relates to a polymeric molding compound made from a first ethylene polymer (A) and from a second ethylene polymer (B) which is particularly suitable for producing thick-walled large-caliber pipes, wherein the molding compound comprises an amount in the range from 55 to 75% by weight of the first ethylene polymer (A) and an amount in the range from 25 to 45% by weight of the second ethylene polymer (B), based in each case on the total weight of the molding compound, where the first ethylene polymer (A) is a copolymer of ethylene with a 1-olefin having a total number of carbon atoms in the range from 4 to 10 as comonomer, and with a proportion of from 0.2 to 5% by weight of comonomer, based on the weight of the first ethylene polymer (A), and with a wide bimodal molar mass distribution, and where the second ethylene polymer (B) is a copolymer made from ethylene units and from a 1-olefin having a number of carbon atoms in the range from 4 to 10, which has a bimodal molar mass distribution differing from that of the first ethylene polymer (A). The invention further relates to a high-strength pipe made from this molding compound, and to its use for the transport of gas or water.

[0001] The present invention relates to a polymeric molding compoundmade from a first ethylene polymer (A) and from a second ethylenepolymer (B). The processing properties of the molding compound make itparticularly suitable for producing thick-walled, large-caliber pipes.

[0002] Polyethylene is widely used for producing pipes, e.g. for gastransport or water transport systems, because pipes of this type requirea material with high mechanical strength, high corrosion resistance, andgood long-term resistance. Numerous publications describe materials witha very wide variety of properties, and processes for their preparation.

[0003] EP-A-603,935 has previously described a molded compound based onpolyethylene and having a bimodal molar mass distribution, and intended,inter alia, to be suitable for producing pipes. However, pipes producedfrom the molding compounds of that reference are highly unsatisfactoryin relation to their long-term resistance to internal pressure, theirstress-cracking resistance, their low-temperature notch impact strength,and their resistance to rapid crack propagation.

[0004] In order to obtain pipes with balanced mechanical properties andtherefore with an ideal combination of properties, it is necessary touse a polymer with still broader molar mass distribution. A polymer ofthis type has been described in U.S. Pat. No. 5,338,589, and is preparedusing a high-activity Ziegler catalyst which is known from WO 91/18934,the magnesium alkoxide used there being a gel-type suspension.

[0005] A disadvantage with the processing of the known molding compoundsis that their melt strength is too low. This becomes noticeableparticularly during processing to give pipes. A specific risk apparentduring that process is that the pipe breaks open while molten or duringconsolidation of the pipe, e.g. in a vacuum calibrator unit. Inaddition, the low melt strength frequently leads to continuousinstability of the extrusion process. Furthermore, when the knownmolding compounds are processed a problem of sagging arises duringextrusion of thick-walled pipes. The problem is that specified thicknesstolerances cannot be complied with during industrial manufacture sincethe total time required for consolidation of the pipes fromthermo-plastic is up to a number of hours and the dead weight of themelt therefore causes uneven wall thickness measured around the entirecircumference of the pipes.

[0006] It was therefore an object of the invention to provide apolyethylene molding compound which has sufficiently high melt strengthto permit its use for producing large-caliber, thick-walled pipes withno risk of break-open of the pipes during production or of the problemof sagging, but at the same time with mechanical properties and producthomogeneity which are sufficient to comply with the quality criteria forthe pipes, such as long-term resistance to internal pressure, highstress-cracking resistance, low-temperature notch impact strength, andhigh resistance to rapid crack propagation.

[0007] This object is achieved by way of a molding compound of the typestated at the outset, the characterizing features of which are that themolding compound comprises an amount in the range from 55 to 75% byweight of the first ethylene polymer (A) and an amount in the range from25 to 45% by weight of the second ethylene polymer (B), based in eachcase on the total weight of the molding compound, where the firstethylene polymer (A) is a copolymer of ethylene with a 1-olefin having atotal number of carbon atoms in the range from 4 to 10 as comonomer, andwith a proportion of from 0.2 to 5% by weight of comonomer, based on theweight of the first ethylene polymer (A), with a wide bimodal molar massdistribution, and where the second ethylene polymer (B) is a copolymermade from ethylene and from a 1-olefin having a number of carbon atomsin the range from 4 to 10, which has a bimodal molar mass distributiondiffering from that of the first ethylene polymer (A).

[0008] The molding compound of the invention is prepared by mixing thecomponents of the mixture, prepared separately from one another, thefirst ethylene polymer (A) and the second ethylene polymer (B), in anextruder in the form of an extruder blend.

[0009] The molding compound of the invention, which can be used tomanufacture a pipe in compliance with the demanding quality criteria onwhich the object of the invention is based, preferably comprises a firstethylene polymer (A) with a density (measured at a temperature of 23°C.) in the range from 0.94 to 0.96 g/cm³ and comprises a broad bimodalmolar mass distribution, where the ratio, within the ethylene polymer(A), between the weight of the low-molecular-weight fraction and theweight of the higher-molecular-weight fraction is in the range from 0.5to 2.0, preferably from 0.8 to 1.8. According to the invention, thefirst ethylene polymer (A) contains small proportions of other comonomerunits, such as 1-butene, 1-pentene, 1-hexene, or 4-methyl-1-pentene.

[0010] The bimodality of the first ethylene polymer (A) may be describedas a measure of the position of the centers of gravity of two individualmolar mass distributions, with the aid of the viscosity numbers VN toISO/R 1191 of the polymers formed in two separate polymerization stages.VN₁ of the low-molecular-weight polyethylene formed in the firstpolymerization stage here is from 40 to 80 cm³/g, whereas VN_(total) ofthe final product is in the range from 350 to 450 cm³/g. VN₂ of thehigher-molecular-weight polyethylene formed in the second polymerizationstage can be calculated from the following mathematical formula:${VN}_{2} = \frac{{VN}_{total} - {w_{1} \cdot {VN}_{1}}}{1 - w_{1}}$

[0011] where w₁ is the proportion by weight of the low-molecular-weightpolyethylene formed in the first stage, measured in % by weight, basedon the total weight of the polyethylene formed in both stages and havingbimodal molar mass distribution. The value calculated for VN₂ isnormally in the range from 500 to 880 cm³/g.

[0012] The first ethylene polymer (A) is obtained by polymerizing themonomers in suspension, in solution, or in the gas phase, attemperatures in the range from 20 to 120° C., at a pressure in the rangefrom 2 to 60 bar, and in the presence of a Ziegler catalyst composed ofa transition metal compound and of an organoaluminum compound. Thepolymerization is carried out in two stages, hydrogen being used in eachstage to regulate the molar mass of the polymer produced.

[0013] According to the invention, therefore, a first ethylene polymer(A) is prepared and contains an amount in the range from 35 to 65% byweight of low-molecular-weight homopolymer as component (A¹), andcontains an amount in the range from 65 to 35% by weight ofhigh-molecular-weight copolymer as component (A²), based on the totalweight of the first ethylene polymer (A).

[0014] The low-molecular-weight homopolymer of component (Al) here has aviscosity number VN^(A1) in the range from 40 to 90 cm³/g, and has anMFRA^(A1) _(190/2.16) in the range from 40 to 2 000 dg/min. According tothe invention, the density d^(A1) of the low-molecular-weighthomopolymer of component (A¹) is in the range up to a maximum of 0.965g/cm³.

[0015] In contrast, the high-molecular-weight copolymer of component(A²) has a viscosity number VN^(A2) in the range from 500 to 1 000 cm³/gand a density d^(A2) in the range from 0.922 to 0.944 g/cm³.

[0016] A very useful tool for determining comonomer distribution insemicrystalline polyethylene is preparative TREF (Temperature-RisingElution Fractionation). This is described in Polym. Prep. A, Chem.Soc.—Polym. Chem. Div., 18, 182 (1977) by L. Wild and T. Ryle under thetitle: “Crystallization distribution in Polymers: A new analyticaltechnique”. This fractionating method is based on the different abilityof the individual components of a polymer to crystallize inpolyethylene, and therefore permits the semicrystalline polymer to beseparated into various fractions which are simply a function of thethickness of the crystallite lamellae.

[0017]FIG. 1 shows the result of a gel-permeation chromatography studyof a TREF fraction at 78° C. of a copolymer typically used as firstethylene polymer (A) for the molding compound of the invention.

[0018] The peak indicated by reference numeral 1 covers thelow-molecular-weight, but highly crystalline, PE fraction, soluble at78° C., while the peak with reference numeral 2 represents thehigh-molecular-weight fraction with high comonomer content, thisfraction being responsible for the large number of “tie molecules”between the crystallite lamellae and for the quality of the moldingcompound of the invention, expressed in terms of its extremely highstress-cracking resistance. The high-molecular-weight copolymer ofcomponent (A²) in the fraction at a temperature of 78° C. frompreparative TREF therefore has an average molar mass, expressed in termsof the weight average M_(w), of ≧180 000 g/mol.

[0019] The second ethylene polymer (B) present in the molding compoundof the invention is a copolymer of ethylene which likewise has a bimodalmolar mass distribution and has an MFR^(B) _(190/5) in the range from0.09 to 0.19 dg/min, a density d^(B) in the range from 0.94 to 0.95g/cm³, and a viscosity number VN^(B) in the range from 460 to 520 cm³/g.

[0020] According to the invention, therefore, a second ethylene polymer(B) is prepared in the form of a reactor blend in the presence of aZiegler catalyst, and comprises an amount in the range from 15 to 40% byweight of ultrahigh-molecular-weight ethylene homo-polymer as component(B¹) and comprises an amount in the range from 60 to 85% by weight oflow-molecular-weight copolymer with 1-butene, 1-hexene, or 1-octene ascomonomer in an amount of from 1 to 15% by weight, as component (B²),based on the total weight of the second ethylene polymer (B). Theultrahigh-molecular-weight ethylene homopolymer of component (B¹) herehas a viscosity number, VN^(B1), in the range from 1 000 to 2 000 cm³/g,and the low-molecular-weight copolymer of component (B²) has a viscositynumber, VN^(B2), in the range from 80 to 150 cm³/g.

[0021] The molding compound of the invention for the pipe to be producedmay also comprise other additives besides the first ethylene polymer (A)and the second ethylene polymer (B). Examples of these additives areheat stabilizers, antioxidants, UV absorbers, light stabilizers, metaldeactivators, compounds which decompose peroxides, or basiccostabilizers, in amounts of from 0 to 10% by weight, preferably from 0to 5% by weight, and also fillers, reinforcing agents, plasticizers,lubricants, emulsifiers, pigments, optical brighteners, flameretardants, antistats, blowing agents, or combinations of these, intotal amounts of from 0 to 50% by weight, based on the total weight ofthe molding compound.

[0022] The manner of producing the pipe from the molding compound of theinvention is that the molding compound is first plastified in anextruder at temperatures in the range from 200 to 250° C. and is thenextruded through an annular die and cooled. Pipes made from the moldingcompound of the invention are generally suit-able for all pressureclasses to DIN 8074.

[0023] For processing to give pipes, use may be made either ofconventional single-screw extruders with smooth feed zone or ofhigh-performance extruders which have a finely grooved barrel and have afeed with conveying action. The screws are typically designed asdecompression screws with lengths from 25 to 30 D (D=Ø). Thedecompression screws have a metering zone in which temperaturedifferences within the melt are evened out, and in which the intentionis to dissipate the relaxation stresses produced by shear.

[0024] The melt coming from the extruder is first distributed by way ofconically arranged holes around an annular cross section, and then fedby way of a spiral mandrel distributor or screen pack to the mandrel/diering combination. When required, there may also be restrictor rings orother design elements installed to render the melt stream uniform priorto die discharge.

[0025] Vacuum calibration is advantageously used for calibration andcooling to give large pipe diameters. The actual shaping process takesplace using slotted calibrator sleeves, manufactured from non-ferrousmetal to improve heat dissipation. A film of water introduced within theinlet serves here for rapid cooling of the surface of the pipe to belowthe crystallite melting point, and also serves as a lubricating film forreducing frictional forces. The total length L of the cooling section isjudged on the basis of the assumption that the intention is that a meltwhose temperature is 220° C. is to be cooled with the aid of water whosetemperature is from 15 to 20° C. sufficiently for the temperature of theinner surface of the pipe to be not more than 85° C.

[0026] Stress-cracking resistance is a feature known previously fromEP-A 436 520. The process of slow crack propagation can be substantiallyinfluenced via molecular structural parameters, such as molar massdistribution and comonomer distribution. The number of what are calledtie molecules or link molecules is first determined by the chain lengthof the polymer. The morphology of semicrystalline polymers is alsoadjusted by incorporating comonomers, since the thickness of crystallitelamellae can be influenced by introducing short-chain branching. Thismeans that the number of what are known as tie molecules or linkmolecules in copolymers is higher than in homopolymers having comparablechain lengths.

[0027] Stress-cracking resistance FNCT of the molding compound of theinvention is determined by an internal test method. This laboratorymethod has been described by M. Fleiβner in Kunststoffe 77 (1987), pp.45 et seq. This publication shows that there is a relationship betweenthe determination of slow crack propagation in the long-term test ontest specimens with a peripheral notch and the brittle variant of thelong-term hydrostatic strength test to ISO 1167. The notch (1.6 mm,razor blade) shortens crack-initiation time and thus time-to-failure in2% strength aqueous Arkopal N 100 detergent solution acting asstress-crack-promoting medium at a temperature of 95° C. and withtensile stress of 4.0 MPa. The specimens are produced by sawing threetest specimens of dimensions 10×10×90 mm from a pressed plaque ofthickness 10 mm. A razor blade in a notching apparatus (see FIG. 5 inthe Fleiβner publication) specifically made for the purpose is used togive the center of the test specimens a peripheral notch of depth 1.6mm.

[0028] Fracture toughness aFM of the molding compound of the inventionis likewise determined by an internal test method on test specimens ofdimensions 10×10×80 mm, sawn out from a pressed plaque of thickness 10mm. The razor blade of the abovementioned notching apparatus is used togive six of these test specimens a central notch of depth 1.6 mm. Themethod of carrying out the tests substantially corresponds to the ISO179 Charpy test procedure with modified test specimens and modifiedimpact geometry (distance between supports). All of the test specimensare conditioned to the test temperature of 0° C. for from 2 to 3 h. Atest specimen is then moved without delay onto the support of a pendulumimpact tester to ISO 179. The distance between the supports is 60 mm.The 2 J hammer is released and falls, with the angle of fall adjusted to160° C., the pendulum length to 225 mm, and the impact velocity to 2.93m/sec. To evaluate the test, the quotient in mJ/mm² is calculated fromthe impact energy consumed and the initial cross-sectional area at thenotch a_(FM). The only values here which can be used as the basis for anoverall average are those for complete fracture and hinge fracture (seeISO 179).

[0029] Shear viscosity is a very particularly important feature of thepolymer melt and represents the flow properties of the polymer extrudedin molten form to give a pipe, these properties being very decisiveaccording to the invention. It is measured to ISO 6721-10, part 10, inoscillating shear flow in a cone-plate rheometer (RDS test) initially atangular frequency of 0.001 rad/s and melt temperature 190° C., and thenat angular frequency 100 rad/s at the same temperature. The two valuesmeasured are then placed in relationship to one another, giving theviscosity ratio η(0.001)/η(100), which according to the invention is tobe greater than or equal to 100.

[0030] The examples below are intended for further clarification of thedescription of the invention and its advantages for the skilled worker,in comparison with the prior art.

EXAMPLES 1 TO 9

[0031] A first bimodal ethylene polymer (A) was prepared to thespecification of WO 91/18934 using a Ziegler catalyst from example 2,which had catalyst component a with operating number 2.2, maintainingthe operating conditions stated below in table 1. TABLE 1 Reactor IReactor II Capacity: 120 l Capacity: 120 l Temperature 83° C. 83° C.Catalyst feed 0.8 mmol/h ---- Cocatalyst feed 15 mmol/h 30 mmol/hDispersing agent 25 l/h 50 l/h (diesel oil; 130-170° C.) Ethylene 9 kg/h10 kg/h Hydrogen in gas space 74% by volume 1% by volume 1-Butene 0 250ml/h Total pressure 8.5 bar 2.7 bar

[0032] The resultant ethylene polymer (A) had a melt flow index MFI^(A)_(5/190° C.) of 0.49 dg/min and a density d^(A) of 0.948 g/cm³, and hada comonomer proportion of 1.5% by weight, based on the total weight ofthe higher-molecular-weight component.

[0033] A second bimodal ethylene polymer (B) was then prepared to thespecification of EP-B-0 003 129. For this, 6.7 kg of ethylene/h and 0.24kg of 1-butene/h were introduced into diesel oil with boiling point inthe range from 130 to 170° C. in a stirred tank over a period of 6 h ata constant temperature of 85° C., in the presence of the Zieglercatalyst described in example 1 of the EP-B. After a reaction time of 3h and 20 min, hydrogen was also introduced under pressure and itsaddition was continued so as to maintain a constant hydrogenconcentration in the region of 60-65% by volume within the gas space ofthe stirred tank during the remaining reaction time of 2 h and 40 min.

[0034] The resultant ethylene polymer (B) had a melt flow index MFI^(B)_(5/190° C.) of 0.16 dg/min and a density dB of 0.940 g/cm³.

[0035] The first bimodal ethylene polymer (A) was then mixed with thesecond bimodal ethylene polymer (B) in an extruder.

[0036] The mixing ratios are given in the table given below for examples1 to 9, as are the attendant physical properties of the molding compoundresulting from the mixture: TABLE 2 Example No: 1 2 3 4 5 6 7 8 9 % byweight of 0 15 20 25 30 35 40 45 100 polymer (B) % by weight of 100 8580 75 70 65 60 55 0 polymer (A) MFI 190/5 0.49 0.31 0.3 0.295 0.285 0.270.26 0.24 0.16 [dg/min] MFI 190/21.6 7.845 6.635 7.72 6.28 6.21 5.8956.04 6.03 4.88 [dg/min] FRR* 16.0 21.4 25.7 21.3 21.8 21.8 23.2 25.137.5 VN [cm³/g] 344 359 357 356 377 395 390 393 486

[0037] The shear viscosities η of the mixtures of examples 1 to 9 weredetermined by the test method described above (ISO 6721, part 10), withangular frequency of 0.001 rad/s and angular frequency of 100 rad/s, andthe ratio η_(0.001 r/s)/η_(100 r/s) was then calculated. The results aregiven in table 3 below: Table 3 TABLE 3 η (0.001 rad/s) η (100 rad/s) η(0.001 rad/s) / Example [Pa · s] [Pa · s] η (100 rad/s) 1 2.25 · 10⁵2450 91.8 2 2.28 · 10⁵ 2500 91.2 3 2.32 · 10⁵ 2556 90.7 4 2.78 · 10⁵2530 109.8 5 2.76 · 10⁵ 2570 107.4 6 3.55 · 10⁵ 2540 139.8 7 4.02 · 10⁵2550 157.6 8 4.86 · 10⁵ 2550 190.6 9 11.6 · 10⁵ 2720 426.5

[0038] A glance at table 3 shows that the mixtures of examples 1 to 3are comparative examples in which the ratio of the shear viscositiesη_(0.001 r/s)/η_(100 r/s) determined at different angular frequencies isbelow 100. In contrast, examples 4 to 8 have results according to theinvention, and for these examples the ratio by weight of polymer (A) topolymer (B) is also in the range according to the invention, from 55 to75% by weight of polymer (A) and from 25 to 45% by weight of polymer B.

EXAMPLES 10 TO 12

[0039] To determine the homogeneity of the mixture (freedom fromspecks), the following three further molding compounds were prepared:

[0040] Example 10 was the molding compound from example 1, i.e. purepolymer (A).

[0041] Example 11 was an in-situ reactor blend, i.e. a modified polymer(A), in which the amounts of ethylene in reactor 1 and reactor 2 wereswapped during the production process. 10 kg of ethylene/h were addedwithin reactor 1, and only 9 kg of ethylene/h within reactor 2, plus 260ml/h of 1-butene as comonomer. The resultant modified polymer (A) had anMFI^(A′) _(5/190° C.) of 0.33 dg/min, and a density of 0.956 g/cm³, andcontained an amount of 1.7% by weight of comonomer, based on the totalweight of the higher-molecular-weight component.

[0042] Example 12 was a mixture made from 34% by weight of polymer (B)and 66% of polymer (A).

[0043] Polymer powder from examples 10 and 11 was pelletized in anextruder and then processed to give blown films of thickness 5 μm Themixture of example 12 made from the powders of the polymers (A) and (B)was then prepared in the same extruder at the same temperature and thesame output rate, and further processed by a similar method. The shearviscosities η of these molding compounds were then measured at thedifferent angular frequencies and their relationship determined, andhomogeneity (freedom from specks) was tested. The results from examples10 to 12 are given in table 4 below: TABLE 4 Homogeneity to η (10⁻³rad/s η (100 rad/s) η_(0.001 r/s)/ GKR guideline, Example [Pa · s] [Pa ·s] η_(100 r/s) max. size^(*)) 10 1.70 · 10⁵ 2570 66.1 0.013 11 2.55 ·10⁵ 2400 106.3 0.014 12 3.75 · 10⁵ 1980 146 0.0010

[0044] Other properties of the polymers prepared in examples 10 to 12are given in table 5 below. TABLE 5 Density MFR_(190/21.6) Viscositynumber Example [g/cm³] [dg/min] [ml/g] 10 0.954 9.2 330 11 0.956 9.52370 12 0.954 8.8 340

[0045] It is entirely surprising to the skilled worker that a suddenimprovement in homogeneity and freedom from specks is given, at the sametemperature and the same throughput rate, only by the mixture of theinvention.

[0046] The test methods given in the description prior to the exampleswere then also used to determine FNCT stress-cracking resistance [h] ata temperature of 95° C., and fracture toughness aFM [kJ/m²] at atemperature of 0° C. The results are given in table 6 below: TABLE 6 aFM[kJ/m²] FNCT [h] Example 10 8.9 not determined Example 11 8.1 130.1Example 12 10.6 175.0

[0047] Here again, it is clear that a step increase in FNCTstress-cracking resistance and, together with this, also a step increasein fracture toughness aFM are given only by the mixture of the inventionmade from ethylene polymer A and ethylene polymer B in the mixing ratiofound according to the invention.

What is claimed is:
 1. A polymeric molding compound made from a firstethylene polymer (A) and from a second ethylene polymer (B) which isparticularly suitable for producing thick-walled large-caliber pipes,wherein the molding compound comprises an amount in the range from 55 to75% by weight of the first ethylene polymer (A) and an amount in therange from 25 to 45% by weight of the second ethylene polymer (B), basedin each case on the total weight of the molding compound, where thefirst ethylene polymer (A) is a copolymer of ethylene with a 1-olefinhaving a total number of carbon atoms in the range from 4 to 10 ascomonomer, and with a proportion of from 0.2 to 5% by weight ofcomonomer, based on the weight of the first ethylene polymer (A), with awide bimodal molar mass distribution, and where the second ethylenepolymer (B) is a copolymer made from ethylene units and from a 1-olefinhaving a number of carbon atoms in the range from 4 to 10, which has abimodal molar mass distribution differing from that of the firstethylene polymer (A).
 2. The polymeric molding compound as claimed inclaim 1, which is prepared by mixing the mixing components, preparedseparately from one another, the first ethylene polymer (A) and thesecond ethylene polymer (B), in an extruder in the form of an extruderblend.
 3. The polymeric molding compound as claimed in claim 1 or 2,which preferably comprises a first ethylene polymer (A) with a density(measured at a temperature of 23° C.) in the range from 0.94 to 0.96g/cm³ and comprises a broad bimodal molar mass distribution, where theratio, within the ethylene polymer (A), between the weight of thelow-molecular-weight fraction and the weight of thehigher-molecular-weight fraction is in the range from 0.5 to 2.0,preferably from 0.8 to 1.8.
 4. The polymeric molding compound as claimedin any of claims 1 to 3, wherein the first ethylene polymer (A) containsan amount from 0.2 to 4.5% by weight of other comonomer units selectedfrom the group consisting of 1-butene, 1-pentene, 1-hexene,4-methyl-1-pentene, and mixtures of these.
 5. The polymeric moldingcompound as claimed in any of claims 1 to 4, which comprises, based onthe total weight of the second ethylene polymer (B), which has beenprepared in the form of a reactor blend in the presence of a Zieglercatalyst, and which comprises an amount in the range from 15 to 40% byweight of ultrahigh-molecular-weight ethylene homopolymer as component(B¹) and comprises an amount in the range from 60 to 85% by weight oflow-molecular-weight copolymer with 1-butene as comonomer in an amountof from 1 to 15% by weight, as component (B²).
 6. The polymeric moldingcompound as claimed in claim 5, wherein the ultrahigh-molecular-weightethylene homopolymer of component (B¹) has a viscosity number, VN^(B1),in the range from 1 000 to 2 000 cm³/g, and wherein thelow-molecular-weight homopolymer of component (B²) has a viscositynumber, VN^(B2), in the range from 80 to 150 cm³/g.
 7. The polymericmolding compound as claimed in any of claims 1 to 6, which has fracturetoughness aFM greater than or equal to 10 kJ/m².
 8. The polymericmolding compound as claimed in any of claims 1 to 7, which has an FNCTstress-cracking resistance of ≧150 h.
 9. The polymeric molding compoundas claimed in any of claims 1 to 8, whose shear viscosity, measured at0.001 rad/s, is ≧2.0·10⁵ Pa·s, preferably ≧2.7·10⁵ Pa·s.
 10. Thepolymeric molding compound as claimed in any of claims 1 to 9, whoseviscosity ratio of the shear viscosities of the molding compoundη_((0.001))/η₍₁₀₀₎ is greater than or equal to
 100. 11. A high-strengthpipe made from a molding compound as claimed in any of claims 1 to 10,wherein the ethylene polymer A contains comonomers having from 4 to 6carbon atoms, the amount being from 0 to 0.1% by weight in thelow-molecular-weight fraction and from 2.5 to 4% by weight in thehigher-molecular-weight fraction, and has a melt flow indexMFI₅/_(190° C.) of ≦0.35 g/10 min.
 12. The pipe as claimed in claim 10,whose resistance to rapid crack propagation, measured to ISO/DIS 13477on a pipe in pressure class PN 10 with diameter 110 mm (S4 test) isgreater than or equal to 20 bar.
 13. The use of a pipe as claimed inclaim 11 or 12 for the transport of gases, and in particular for thetransport of natural gas, or water. from 0.94 to 0.96 g/cm³ andcomprises a broad bimodal molar mass distribution, where the ratio,within the ethylene polymer (A), between the weight of thelow-molecular-weight fraction and the weight of thehigher-molecular-weight fraction is in the range from 0.5 to 2.0. 15.The polymeric molding compound as claimed in claim 2, which comprises afirst ethylene polymer (A) with a density (measured at a temperature of23° C.) in the range from 0.94 to 0.96 g/cm³ and comprises a broadbimodal molar mass distribution, where the ratio, within the ethylenepolymer (A), between the weight of the low-molecular-weight fraction andthe weight of the higher-molecular-weight fraction is in the range from0.8 to 1.8.
 16. The polymeric molding compound as claimed in claim 1,wherein the first ethylene polymer (A) contains an amount from 0.2 to4.5% by weight of other comonomer units selected from the groupconsisting of 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, andmixtures of these.
 17. The polymeric molding compound as claimed inclaim 15, wherein the first ethylene polymer (A) contains an amount from0.2 to 4.5% by weight of other comonomer units selected from the groupconsisting of 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, andmixtures of these.
 18. The polymeric molding compound as claimed inclaim 1, which comprises, based on the total weight of the secondethylene polymer (B), which has been prepared in the form of a reactorblend in the presence of a Ziegler catalyst, and which comprises anamount in the range from 15 to 40% by weight ofultrahigh-molecular-weight ethylene homopolymer as component (B¹) andcomprises an amount in the range from 60 to 85% by weight oflow-molecular-weight copolymer with 1-butene as comonomer in an amountof from 1 to 15% by weight, as component (B²).
 19. The polymeric moldingcompound as claimed in claim 17, which comprises, based on the totalweight of the second ethylene polymer (B), which has been prepared inthe form of a reactor blend in the presence of a Ziegler catalyst, andwhich comprises an amount in the range from 15 to 40% by weight ofultrahigh-molecular-weight ethylene homopolymer as component (B¹) andcomprises an amount in the range from 60 to 85% by weight oflow-molecular-weight copolymer with 1-butene as comonomer in an amountof from 1 to 15% by weight, as component (B²).
 20. The polymeric moldingcompound as claimed in claim 18, wherein the ultrahigh-molecular-weightethylene homopolymer of component (B¹) has a viscosity number, VN^(B1),in the range from 1 000 to 2 000 cm³/g, and wherein thelow-molecular-weight homopolymer of component (B²) has a viscositynumber, VN^(B2), in the range from 80 to 150 cm³/g.
 21. The polymericmolding compound as claimed in claim 1, wherein the molding compound hasfracture toughness aFM greater than or equal to 10 kJ/m².
 22. Thepolymeric molding compound as claimed in claim 1, wherein the moldingcompound has an FNCT stress-cracking resistance of ≧150 h.
 23. Thepolymeric molding compound as claimed in claim 1, wherein the moldingcompound has shear viscosity, measured at 0.001 rad/s, is ≧2.0·10⁵ Pa·s.24. The polymeric molding compound as claimed in claim 19, wherein themolding compound has shear viscosity, measured at 0.001 rad/s, is≧2.7·10⁵ Pa·s.
 25. The polymeric molding compound as claimed in claim 1,wherein the molding compound the viscosity ratio of the shearviscosities of the molding compound η_((0.001))/η₍₁₀₀₎ is greater thanor equal to
 100. 26. A high-strength pipe made from the molding compoundas claimed in claim 1, wherein the ethylene polymer A containscomonomers having from 4 to 6 carbon atoms, the amount being from 0 to0.1% by weight in the low-molecular-weight fraction and from 2.5 to 4%by weight in the higher-molecular-weight fraction, and has a melt flowindex MFI₅/_(190° C.) of ≦0.35 g/10 min.
 27. The pipe as claimed inclaim 26, wherein the pipe has a resistance to rapid crack propagation,measured to ISO/DIS 13477 on a pipe in pressure class PN 10 withdiameter 110 mm (S4 test) is greater than or equal to 20 bar.
 28. Amethod of transporting gases or water which comprises transporting thegases or the water through the pipe as claimed in claim 26.